Contact Angle Measurements
The wettability of a particular solid coating, substrate or pigment on a surface is a key to its effectiveness, as wetting is important to adhesion as well as to the prevention of surface defects such as craters, dewetting and crawling. A new ASTM International standard uses contact angle measurements to determine the wettability of coatings.
The new standard, D7490, Test Method for Measurement of the Surface Tension of Solid Coatings, Substrates and Pigments Using Contact Angle Measurements, was developed by Subcommittee D01.23 on Physical Properties of Applied Paint Films, part of ASTM International Committee D01 on Paint and Related Coatings, Materials and Applications.
Clifford Schoff, Ph.D., retired from PPG Industries Inc., and a longtime member of Committee D01, says that D7490 will be used to determine the wettability of a given primer or substrate, or it can be used to explain why there are wetting problems or defects over a certain primer or substrate.
“The standard can also be used to characterize pigments that have been pressed into disks and to determine how easy the pigment is to wet,” says Schoff. “This is useful in deciding whether a given dispersing resin solution or paint vehicle will wet the pigment or explaining why it does not.”
According to Schoff, the technique used for contact angle measurement is the same as that described in another D01.23 standard, D7334, Practice for Surface Wettability of Coatings, Substrates and Pigments by Advancing Contact Angle Measurement.
“We built on practice D7334 and developed D7490 for measuring the surface tensions of solid coatings and related materials,” says Schoff. “The key to the test method is that liquids that wet a surface flow out and give a low contact angle whereas those that do not wet bead up and give a high contact angle.”
D7490 will be used by paint formulators, problem solvers, service people and users. In addition, suppliers of additives for the improvement of wetting will find useful applications for the new standard.
Technical Information: Clifford Schoff, Ph.D., Allison Park, Pa.
ASTM Staff: Jeffrey Adkins